Definition
A "plugged" golf ball is one that makes an indentation into the ground and rests in that indentation. Except for in hazards, golfers get free relief from this.
Usage
Your ball plugged in the bunker.
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A plugged lie is when the ball hits the ground hard, makes an indentation in the ground, and then sits in the indentation.
When the ball is in the fairway or rough and is "plugged", you get free relief (no penalty) and get to take it out.
When the ball is "plugged" in a sand trap, we call that a "fried egg". A fried egg is one of the hardest shots to hit in golf because you have to swing hard and move a lot of sand to get the ball in the air.
Unlike a plugged lie in the rough or fairway, with a fried egg, you cannot get free relief and must play it as it lies.